// Copyright 2011 The Snappy-Go Authors. All rights reserved.
// Copyright (c) 2019 Klaus Post. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package s2

import (
	"crypto/rand"
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"math"
	"math/bits"
	"runtime"
	"sync"
)

// Encode returns the encoded form of src. The returned slice may be a sub-
// slice of dst if dst was large enough to hold the entire encoded block.
// Otherwise, a newly allocated slice will be returned.
//
// The dst and src must not overlap. It is valid to pass a nil dst.
//
// The blocks will require the same amount of memory to decode as encoding,
// and does not make for concurrent decoding.
// Also note that blocks do not contain CRC information, so corruption may be undetected.
//
// If you need to encode larger amounts of data, consider using
// the streaming interface which gives all of these features.
func Encode(dst, src []byte) []byte {
	if n := MaxEncodedLen(len(src)); n < 0 {
		panic(ErrTooLarge)
	} else if len(dst) < n {
		dst = make([]byte, n)
	}

	// The block starts with the varint-encoded length of the decompressed bytes.
	d := binary.PutUvarint(dst, uint64(len(src)))

	if len(src) == 0 {
		return dst[:d]
	}
	if len(src) < minNonLiteralBlockSize {
		d += emitLiteral(dst[d:], src)
		return dst[:d]
	}
	n := encodeBlock(dst[d:], src)
	if n > 0 {
		d += n
		return dst[:d]
	}
	// Not compressible
	d += emitLiteral(dst[d:], src)
	return dst[:d]
}

// EncodeBetter returns the encoded form of src. The returned slice may be a sub-
// slice of dst if dst was large enough to hold the entire encoded block.
// Otherwise, a newly allocated slice will be returned.
//
// EncodeBetter compresses better than Encode but typically with a
// 10-40% speed decrease on both compression and decompression.
//
// The dst and src must not overlap. It is valid to pass a nil dst.
//
// The blocks will require the same amount of memory to decode as encoding,
// and does not make for concurrent decoding.
// Also note that blocks do not contain CRC information, so corruption may be undetected.
//
// If you need to encode larger amounts of data, consider using
// the streaming interface which gives all of these features.
func EncodeBetter(dst, src []byte) []byte {
	if n := MaxEncodedLen(len(src)); n < 0 {
		panic(ErrTooLarge)
	} else if len(dst) < n {
		dst = make([]byte, n)
	}

	// The block starts with the varint-encoded length of the decompressed bytes.
	d := binary.PutUvarint(dst, uint64(len(src)))

	if len(src) == 0 {
		return dst[:d]
	}
	if len(src) < minNonLiteralBlockSize {
		d += emitLiteral(dst[d:], src)
		return dst[:d]
	}
	n := encodeBlockBetter(dst[d:], src)
	if n > 0 {
		d += n
		return dst[:d]
	}
	// Not compressible
	d += emitLiteral(dst[d:], src)
	return dst[:d]
}

// ConcatBlocks will concatenate the supplied blocks and append them to the supplied destination.
// If the destination is nil or too small, a new will be allocated.
// The blocks are not validated, so garbage in = garbage out.
// dst may not overlap block data.
// Any data in dst is preserved as is, so it will not be considered a block.
func ConcatBlocks(dst []byte, blocks ...[]byte) ([]byte, error) {
	totalSize := uint64(0)
	compSize := 0
	for _, b := range blocks {
		l, hdr, err := decodedLen(b)
		if err != nil {
			return nil, err
		}
		totalSize += uint64(l)
		compSize += len(b) - hdr
	}
	if totalSize == 0 {
		dst = append(dst, 0)
		return dst, nil
	}
	if totalSize > math.MaxUint32 {
		return nil, ErrTooLarge
	}
	var tmp [binary.MaxVarintLen32]byte
	hdrSize := binary.PutUvarint(tmp[:], totalSize)
	wantSize := hdrSize + compSize

	if cap(dst)-len(dst) < wantSize {
		dst = append(make([]byte, 0, wantSize+len(dst)), dst...)
	}
	dst = append(dst, tmp[:hdrSize]...)
	for _, b := range blocks {
		_, hdr, err := decodedLen(b)
		if err != nil {
			return nil, err
		}
		dst = append(dst, b[hdr:]...)
	}
	return dst, nil
}

// inputMargin is the minimum number of extra input bytes to keep, inside
// encodeBlock's inner loop. On some architectures, this margin lets us
// implement a fast path for emitLiteral, where the copy of short (<= 16 byte)
// literals can be implemented as a single load to and store from a 16-byte
// register. That literal's actual length can be as short as 1 byte, so this
// can copy up to 15 bytes too much, but that's OK as subsequent iterations of
// the encoding loop will fix up the copy overrun, and this inputMargin ensures
// that we don't overrun the dst and src buffers.
const inputMargin = 8

// minNonLiteralBlockSize is the minimum size of the input to encodeBlock that
// will be accepted by the encoder.
const minNonLiteralBlockSize = 32

// MaxBlockSize is the maximum value where MaxEncodedLen will return a valid block size.
// Blocks this big are highly discouraged, though.
const MaxBlockSize = math.MaxUint32 - binary.MaxVarintLen32 - 5

// MaxEncodedLen returns the maximum length of a snappy block, given its
// uncompressed length.
//
// It will return a negative value if srcLen is too large to encode.
// 32 bit platforms will have lower thresholds for rejecting big content.
func MaxEncodedLen(srcLen int) int {
	n := uint64(srcLen)
	if n > 0xffffffff {
		// Also includes negative.
		return -1
	}
	// Size of the varint encoded block size.
	n = n + uint64((bits.Len64(n)+7)/7)

	// Add maximum size of encoding block as literals.
	n += uint64(literalExtraSize(int64(srcLen)))
	if n > 0xffffffff {
		return -1
	}
	return int(n)
}

var errClosed = errors.New("s2: Writer is closed")

// NewWriter returns a new Writer that compresses to w, using the
// framing format described at
// https://github.com/google/snappy/blob/master/framing_format.txt
//
// Users must call Close to guarantee all data has been forwarded to
// the underlying io.Writer and that resources are released.
// They may also call Flush zero or more times before calling Close.
func NewWriter(w io.Writer, opts ...WriterOption) *Writer {
	w2 := Writer{
		blockSize:   defaultBlockSize,
		concurrency: runtime.GOMAXPROCS(0),
	}
	for _, opt := range opts {
		if err := opt(&w2); err != nil {
			w2.errState = err
			return &w2
		}
	}
	w2.obufLen = obufHeaderLen + MaxEncodedLen(w2.blockSize)
	w2.paramsOK = true
	w2.ibuf = make([]byte, 0, w2.blockSize)
	w2.buffers.New = func() interface{} {
		return make([]byte, w2.obufLen)
	}
	w2.Reset(w)
	return &w2
}

// Writer is an io.Writer that can write Snappy-compressed bytes.
type Writer struct {
	errMu    sync.Mutex
	errState error

	// ibuf is a buffer for the incoming (uncompressed) bytes.
	ibuf []byte

	// wroteStreamHeader is whether we have written the stream header.
	wroteStreamHeader bool
	paramsOK          bool
	better            bool

	blockSize   int
	obufLen     int
	concurrency int
	written     int64
	output      chan chan result
	buffers     sync.Pool
	writerWg    sync.WaitGroup
	writer      io.Writer
	pad         int
}

type result []byte

// err returns the previously set error.
// If no error has been set it is set to err if not nil.
func (w *Writer) err(err error) error {
	w.errMu.Lock()
	errSet := w.errState
	if errSet == nil && err != nil {
		w.errState = err
		errSet = err
	}
	w.errMu.Unlock()
	return errSet
}

// Reset discards the writer's state and switches the Snappy writer to write to w.
// This permits reusing a Writer rather than allocating a new one.
func (w *Writer) Reset(writer io.Writer) {
	if !w.paramsOK {
		return
	}
	// Close previous writer, if any.
	if w.output != nil {
		close(w.output)
		w.writerWg.Wait()
		w.output = nil
	}
	w.errState = nil
	w.ibuf = w.ibuf[:0]
	w.wroteStreamHeader = false
	w.written = 0
	w.writer = writer
	// If we didn't get a writer, stop here.
	if writer == nil {
		return
	}
	// If no concurrency requested, don't spin up writer goroutine.
	if w.concurrency == 1 {
		return
	}

	toWrite := make(chan chan result, w.concurrency)
	w.output = toWrite
	w.writerWg.Add(1)

	// Start a writer goroutine that will write all output in order.
	go func() {
		defer w.writerWg.Done()

		// Get a queued write.
		for write := range toWrite {
			// Wait for the data to be available.
			in := <-write
			if len(in) > 0 {
				if w.err(nil) == nil {
					// Don't expose data from previous buffers.
					in = in[:len(in):len(in)]
					// Write to output.
					n, err := writer.Write(in)
					if err == nil && n != len(in) {
						err = io.ErrShortBuffer
					}
					_ = w.err(err)
					w.written += int64(n)
				}
				if cap(in) >= w.obufLen {
					w.buffers.Put([]byte(in))
				}
			}
			// close the incoming write request.
			// This can be used for synchronizing flushes.
			close(write)
		}
	}()
}

// Write satisfies the io.Writer interface.
func (w *Writer) Write(p []byte) (nRet int, errRet error) {
	// If we exceed the input buffer size, start writing
	for len(p) > (cap(w.ibuf)-len(w.ibuf)) && w.err(nil) == nil {
		var n int
		if len(w.ibuf) == 0 {
			// Large write, empty buffer.
			// Write directly from p to avoid copy.
			n, _ = w.write(p)
		} else {
			n = copy(w.ibuf[len(w.ibuf):cap(w.ibuf)], p)
			w.ibuf = w.ibuf[:len(w.ibuf)+n]
			w.write(w.ibuf)
			w.ibuf = w.ibuf[:0]
		}
		nRet += n
		p = p[n:]
	}
	if err := w.err(nil); err != nil {
		return nRet, err
	}
	// p should always be able to fit into w.ibuf now.
	n := copy(w.ibuf[len(w.ibuf):cap(w.ibuf)], p)
	w.ibuf = w.ibuf[:len(w.ibuf)+n]
	nRet += n
	return nRet, nil
}

// ReadFrom implements the io.ReaderFrom interface.
// Using this is typically more efficient since it avoids a memory copy.
// ReadFrom reads data from r until EOF or error.
// The return value n is the number of bytes read.
// Any error except io.EOF encountered during the read is also returned.
func (w *Writer) ReadFrom(r io.Reader) (n int64, err error) {
	if len(w.ibuf) > 0 {
		err := w.Flush()
		if err != nil {
			return 0, err
		}
	}
	w.ibuf = w.ibuf[:w.blockSize]
	for {
		n2, err := io.ReadFull(r, w.ibuf)
		if err != nil {
			if err == io.ErrUnexpectedEOF {
				err = io.EOF
			}
			if err != io.EOF {
				return n, w.err(err)
			}
		}
		if n2 == 0 {
			break
		}
		n += int64(n2)
		w.ibuf = w.ibuf[:n2]
		n3, err2 := w.write(w.ibuf)
		if w.err(err2) != nil {
			break
		}
		if n3 != n2 {
			return n, w.err(errors.New("internal error: size uncompressed size mismatch"))
		}
		if err != nil {
			// We got EOF and wrote everything
			break
		}
	}
	w.ibuf = w.ibuf[:0]
	return n, w.err(nil)
}

func (w *Writer) write(p []byte) (nRet int, errRet error) {
	if err := w.err(nil); err != nil {
		return 0, err
	}
	if w.concurrency == 1 {
		return w.writeSync(p)
	}

	// Spawn goroutine and write block to output channel.
	for len(p) > 0 {
		if !w.wroteStreamHeader {
			w.wroteStreamHeader = true
			hWriter := make(chan result)
			w.output <- hWriter
			hWriter <- []byte(magicChunk)
		}

		var uncompressed []byte
		if len(p) > w.blockSize {
			uncompressed, p = p[:w.blockSize], p[w.blockSize:]
		} else {
			uncompressed, p = p, nil
		}

		// Copy input.
		// If the block is incompressible, this is used for the result.
		inbuf := w.buffers.Get().([]byte)[:len(uncompressed)+obufHeaderLen]
		obuf := w.buffers.Get().([]byte)[:w.obufLen]
		copy(inbuf[obufHeaderLen:], uncompressed)
		uncompressed = inbuf[obufHeaderLen:]

		output := make(chan result)
		// Queue output now, so we keep order.
		w.output <- output
		go func() {
			checksum := crc(uncompressed)

			// Set to uncompressed.
			chunkType := uint8(chunkTypeUncompressedData)
			chunkLen := 4 + len(uncompressed)

			// Attempt compressing.
			n := binary.PutUvarint(obuf[obufHeaderLen:], uint64(len(uncompressed)))
			var n2 int
			if w.better {
				n2 = encodeBlockBetter(obuf[obufHeaderLen+n:], uncompressed)
			} else {
				n2 = encodeBlock(obuf[obufHeaderLen+n:], uncompressed)
			}

			// Check if we should use this, or store as uncompressed instead.
			if n2 > 0 {
				chunkType = uint8(chunkTypeCompressedData)
				chunkLen = 4 + n + n2
				obuf = obuf[:obufHeaderLen+n+n2]
			} else {
				// Use input as output.
				obuf, inbuf = inbuf, obuf
			}

			// Fill in the per-chunk header that comes before the body.
			obuf[0] = chunkType
			obuf[1] = uint8(chunkLen >> 0)
			obuf[2] = uint8(chunkLen >> 8)
			obuf[3] = uint8(chunkLen >> 16)
			obuf[4] = uint8(checksum >> 0)
			obuf[5] = uint8(checksum >> 8)
			obuf[6] = uint8(checksum >> 16)
			obuf[7] = uint8(checksum >> 24)

			// Queue final output.
			output <- obuf

			// Put unused buffer back in pool.
			w.buffers.Put(inbuf)
		}()
		nRet += len(uncompressed)
	}
	return nRet, nil
}

func (w *Writer) writeSync(p []byte) (nRet int, errRet error) {
	if err := w.err(nil); err != nil {
		return 0, err
	}
	for len(p) > 0 {
		if !w.wroteStreamHeader {
			w.wroteStreamHeader = true
			n, err := w.writer.Write([]byte(magicChunk))
			if err != nil {
				return 0, w.err(err)
			}
			if n != len(magicChunk) {
				return 0, w.err(io.ErrShortWrite)
			}
			w.written += int64(n)
		}

		var uncompressed []byte
		if len(p) > w.blockSize {
			uncompressed, p = p[:w.blockSize], p[w.blockSize:]
		} else {
			uncompressed, p = p, nil
		}

		obuf := w.buffers.Get().([]byte)[:w.obufLen]
		checksum := crc(uncompressed)

		// Set to uncompressed.
		chunkType := uint8(chunkTypeUncompressedData)
		chunkLen := 4 + len(uncompressed)

		// Attempt compressing.
		n := binary.PutUvarint(obuf[obufHeaderLen:], uint64(len(uncompressed)))
		var n2 int
		if w.better {
			n2 = encodeBlockBetter(obuf[obufHeaderLen+n:], uncompressed)
		} else {
			n2 = encodeBlock(obuf[obufHeaderLen+n:], uncompressed)
		}

		if n2 > 0 {
			chunkType = uint8(chunkTypeCompressedData)
			chunkLen = 4 + n + n2
			obuf = obuf[:obufHeaderLen+n+n2]
		} else {
			obuf = obuf[:8]
		}

		// Fill in the per-chunk header that comes before the body.
		obuf[0] = chunkType
		obuf[1] = uint8(chunkLen >> 0)
		obuf[2] = uint8(chunkLen >> 8)
		obuf[3] = uint8(chunkLen >> 16)
		obuf[4] = uint8(checksum >> 0)
		obuf[5] = uint8(checksum >> 8)
		obuf[6] = uint8(checksum >> 16)
		obuf[7] = uint8(checksum >> 24)

		n, err := w.writer.Write(obuf)
		if err != nil {
			return 0, w.err(err)
		}
		if n != len(obuf) {
			return 0, w.err(io.ErrShortWrite)
		}
		w.written += int64(n)
		if chunkType == chunkTypeUncompressedData {
			// Write uncompressed data.
			n, err := w.writer.Write(uncompressed)
			if err != nil {
				return 0, w.err(err)
			}
			if n != len(uncompressed) {
				return 0, w.err(io.ErrShortWrite)
			}
			w.written += int64(n)
		}
		w.buffers.Put(obuf)
		// Queue final output.
		nRet += len(uncompressed)
	}
	return nRet, nil
}

// Flush flushes the Writer to its underlying io.Writer.
// This does not apply padding.
func (w *Writer) Flush() error {
	if err := w.err(nil); err != nil {
		return err
	}

	// Queue any data still in input buffer.
	if len(w.ibuf) != 0 {
		_, err := w.write(w.ibuf)
		w.ibuf = w.ibuf[:0]
		err = w.err(err)
		if err != nil {
			return err
		}
	}
	if w.output == nil {
		return w.err(nil)
	}

	// Send empty buffer
	res := make(chan result)
	w.output <- res
	// Block until this has been picked up.
	res <- nil
	// When it is closed, we have flushed.
	<-res
	return w.err(nil)
}

// Close calls Flush and then closes the Writer.
// Calling Close multiple times is ok.
func (w *Writer) Close() error {
	err := w.Flush()
	if w.output != nil {
		close(w.output)
		w.writerWg.Wait()
		w.output = nil
	}
	if w.err(nil) == nil && w.writer != nil && w.pad > 0 {
		add := calcSkippableFrame(w.written, int64(w.pad))
		frame, err := skippableFrame(w.ibuf[:0], add, rand.Reader)
		if err = w.err(err); err != nil {
			return err
		}
		_, err2 := w.writer.Write(frame)
		err = w.err(err2)
	}
	_ = w.err(errClosed)
	if err == errClosed {
		return nil
	}
	return err
}

const skippableFrameHeader = 4

// calcSkippableFrame will return a total size to be added for written
// to be divisible by multiple.
// The value will always be > skippableFrameHeader.
// The function will panic if written < 0 or wantMultiple <= 0.
func calcSkippableFrame(written, wantMultiple int64) int {
	if wantMultiple <= 0 {
		panic("wantMultiple <= 0")
	}
	if written < 0 {
		panic("written < 0")
	}
	leftOver := written % wantMultiple
	if leftOver == 0 {
		return 0
	}
	toAdd := wantMultiple - leftOver
	for toAdd < skippableFrameHeader {
		toAdd += wantMultiple
	}
	return int(toAdd)
}

// skippableFrame will add a skippable frame with a total size of bytes.
// total should be >= skippableFrameHeader and < maxBlockSize + skippableFrameHeader
func skippableFrame(dst []byte, total int, r io.Reader) ([]byte, error) {
	if total == 0 {
		return dst, nil
	}
	if total < skippableFrameHeader {
		return dst, fmt.Errorf("s2: requested skippable frame (%d) < 4", total)
	}
	if int64(total) >= maxBlockSize+skippableFrameHeader {
		return dst, fmt.Errorf("s2: requested skippable frame (%d) >= max 1<<24", total)
	}
	// Chunk type 0xfe "Section 4.4 Padding (chunk type 0xfe)"
	dst = append(dst, chunkTypePadding)
	f := uint32(total - skippableFrameHeader)
	// Add chunk length.
	dst = append(dst, uint8(f), uint8(f>>8), uint8(f>>16))
	// Add data
	start := len(dst)
	dst = append(dst, make([]byte, f)...)
	_, err := io.ReadFull(r, dst[start:])
	return dst, err
}

// WriterOption is an option for creating a encoder.
type WriterOption func(*Writer) error

// WriterConcurrency will set the concurrency,
// meaning the maximum number of decoders to run concurrently.
// The value supplied must be at least 1.
// By default this will be set to GOMAXPROCS.
func WriterConcurrency(n int) WriterOption {
	return func(w *Writer) error {
		if n <= 0 {
			return errors.New("concurrency must be at least 1")
		}
		w.concurrency = n
		return nil
	}
}

// WriterBetterCompression will enable better compression.
// EncodeBetter compresses better than Encode but typically with a
// 10-40% speed decrease on both compression and decompression.
func WriterBetterCompression() WriterOption {
	return func(w *Writer) error {
		w.better = true
		return nil
	}
}

// WriterBlockSize allows to override the default block size.
// Blocks will be this size or smaller.
// Minimum size is 4KB and and maximum size is 4MB.
//
// Bigger blocks may give bigger throughput on systems with many cores,
// and will increase compression slightly, but it will limit the possible
// concurrency for smaller payloads for both encoding and decoding.
// Default block size is 1MB.
func WriterBlockSize(n int) WriterOption {
	return func(w *Writer) error {
		if w.blockSize > maxBlockSize || w.blockSize < minBlockSize {
			return errors.New("s2: block size too large. Must be <= 4MB and >=4KB")
		}
		w.blockSize = n
		return nil
	}
}

// WriterPadding will add padding to all output so the size will be a multiple of n.
// This can be used to obfuscate the exact output size or make blocks of a certain size.
// The contents will be a skippable frame, so it will be invisible by the decoder.
// n must be > 0 and <= 4MB.
// The padded area will be filled with data from crypto/rand.Reader.
// The padding will be applied whenever Close is called on the writer.
func WriterPadding(n int) WriterOption {
	return func(w *Writer) error {
		if n <= 0 {
			return fmt.Errorf("s2: padding must be at least 1")
		}
		// No need to waste our time.
		if n == 1 {
			w.pad = 0
		}
		if n > maxBlockSize {
			return fmt.Errorf("s2: padding must less than 4MB")
		}
		w.pad = n
		return nil
	}
}
